pwgtoraster: reject overflowing raster buffers

[carrerasdarren-cell]

Summary

• check horizontal row expansion before multiplying the input bytes-per-line
• allocate vertical averaging and planar page buffers with checked factors
• use size_t for averaging and planar row offsets, and handle all three allocation failures before raster decoding

Fixes #192.
Fixes #193.
Fixes #194.

Testing

Built the library with AddressSanitizer and UndefinedBehaviorSanitizer, then ran each issue's exact public Raster fixture through cfFilterPWGToRaster:

• pwgtoraster: horizontal replication undersizes the input row #192 (6a07436a...): rejects the 447,439-byte row before 9,600x expansion
• pwgtoraster: planar page-buffer multiplication wraps to zero #193 (eb3985c5...): rejects the 65,536 x 65,536 planar page before allocation
• pwgtoraster: vertical averaging size wraps before heap allocation #194 (ad3a0b92...): rejects the 512 x 8,388,609 averaging buffer before allocation

All three complete without sanitizer diagnostics. The normal sanitizer control set also passes 3/3 (testdither, testpdf1, and cupsfilters/testfilters.sh), and git diff --check passes.